# zpool upgrade
This system is currently running ZFS pool version 28.
The following pools are out of date, and can be upgraded. After being
upgraded, these pools will no longer be accessible by older software versions.
VER POOL
--- ------------
15 tank
Use 'zpool upgrade -v' for a list of available versions and their associated
features.

So we need to upgrade our pool to the latest version:

# zpool upgrade -a
This system is currently running ZFS pool version 28.
Successfully upgraded 'tank'
If you boot from pool 'tank', don't forget to update boot code.
Assuming you use GPT partitioning and da0 is your boot disk
the following command will do it:
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da0